PURPOSE: To immediately detect the erroneous connection of a cable immediately after the start of a system by detecting a fact that a transmission device being different from its own transmission characteristic is not connected to each interface port, so that an operation of a control part can be executed.

CONSTITUTION: To each SCSI(Small Computer System Interface) port 21, 22, interface cable 11, 12 are connected, and these ports are provided with not only a plus signal line, a minus signal line, etc., but also a DIFFSNS (Differential Sense) line. Each driver 31a, 32a is driven by a driver enable signal, and drives the plus signal line and the minus signal line of the SCSI interface. SCSI control, circuits 31b, 32b control the SCSI through the drivers 31a, 32a, are reset, when a reset signal is applied to an RST terminal, and output the driver enable signal. A transmission characteristic detecting part 40 connects + 5 volts to each DIFFSNS line of the SCSI ports 21, 22, and applies the reset signal to the SCSI control circuits 31b, 32b.